The invention concerns a corrector for chromatic and aperture aberration correction in a scanning electron microscope or a scanning transmission electron microscope, comprising four multipole elements which are consecutively disposed in the optical path, the first and fourth of which generating quadrupole fields, and the second and third of which generating octupole fields and quadrupole fields, wherein the latter are superposed magnetic and electric fields, and wherein the quadrupole fields of all four multipole elements are successively rotated from one to the other through 90°, such that chromatic aberration correction can be performed in the second and third multipole elements using astigmatic intermediate images through interaction between the magnetic and electric fields, and aperture aberration correction can be performed using quadrupole fields and octupole fields.
A corrector of this type changes the beam emitted by the beam source prior to reaching the scanning device after passage through a condenser lens in such a fashion that the chromatic and aperture aberrations of the round lenses of the microscope can be compensated for. This minimizes and exactly defines the scanning point. This is the precondition for generating scanning images of high resolution and high contrast.
The findings of O. Scherzer (O. Scherzer: “Sphärische und chromatische Korrektur von Elektronen-Linsen” (spherical and chromatic correction of electron lenses), OPTIK, DE, JENA, 1947, pages 114-132, XP002090897, ISSN: 0863-0259) form the basis of function of all correctors in particle optics, i.e. that chromatic and aperture aberrations can also be corrected for particle beams by using non-rotationally symmetrical fields that first generate astigmatic intermediate images and then eliminate this astigmatism. O. Scherzer establishes the conditions required to achieve this (cit. loc.). These conditions, which are called the Scherzer theorem, form the basis of any chromatic and aperture aberration correction in particle optics.
Departing therefrom, V. Beck and A. V. Crewe (V. Beck and A. V. Crewe, “A quadrupole-octupole corrector for a 100 keV STEM”, Proceedings 32nd Annual EMSA Meeting, 1974, pages 426, 427) have proposed a corrector for eliminating the aperture aberration, which consists of four quadrupoles with a centrally disposed octupole and one upstream and one downstream octupole. However, this corrector was not able to eliminate chromatic aberrations.
In order to obtain both aperture and chromatic aberration correction, a corrector of the above-mentioned type was constructed on the basis of a proposal by H. Rose (“Abbildungseigenschaften sphärisch korrigierter elektronenmikroskopischer Achromate”) (imaging properties of spherically corrected electron microscopic achromates), Optik 33 (1971), pages 1-24) by J. Zach and M. Haider in correspondence with their publication (“Aberration correction in a low voltage SEM by a multipole corrector”, Nuclear Instruments and Methods in Physics Research A 363 (1995), 316-325). One problem of such correctors is that they cause aberrations themselves, i.e. aberrations such as astigmatism, stellar aberrations, rosette aberrations, and coma, which occur in different orders. These aberrations should be eliminated to as maximum an extent as possible. Moreover, a corrector of the above-mentioned type corrects only aperture aberrations up to third order. The remaining aperture aberration of fifth order is still disturbing.
WO 2007/065382 proposes a corrector with two corrector elements which are anti-symmetric with respect to a middle plane. Each corrector element comprises five multipoles with five quadruple fields and at least one octupole field (three fields are also possible). In an embodiment thereof, a third multipole element disposed in the symmetry plane can be a twelve-pole element generating an octupole field upon which a twelve-pole field is overlaid. A transfer lens system is disposed between the corrector elements and causes the basic path to enter into the first and second corrector element in the same manner.
In contrast to the corrector mentioned above, this corrector does not operate according for the principle of opening and color error corrections in the region of astigmatic intermediate images, since such images are not created. On the contrary, the corrector corrects such errors in conjugated and morphic planes of the two corrector elements. In the symmetry plane of the two corrector elements in which, according to the above mentioned proposal, the octupole field overlaid with a twelve-pole field is produced, the axial beam emanating from an image point does not have a round beam cross-section as is the case in the above mentioned corrector, rather an elliptical beam cross-section, which is disposed outside of the optical axis. In addition to a correction of four fold astigmatisms of third order, this corrector also causes additional distorting beam influences which are, however, substantially harmless, since the anti-symmetric configuration of the second corrector induces an opposite beam influence to thereby compensate for the distortions. A second corrector in anti-symmetric configuration is therefore absolutely indispensable for the transfer lens system. Moreover, the different principle of functioning than for a corrector of the above mentioned kind without generating an astigmatic intermediate image also requires a completely different beam path, as can be seen from FIGS. 1, 2 and 3.
U.S. Pat. No. 7,015,481 b2 is based on a publication due to J. Zach and M. Haider which deals with the utilization of twelve-poles for the production of the fields mentioned above. However, this document does not suggest introduction of additional multipoles rather regulation of the existing multipoles in order to effect further corrections of errors through use of corresponding fields, in particular opening errors of fifth order. However, the Shew configuration (angular displacement of a pole of a multipole element) which is proposed herein for the correction plays a rather indirect role. It would apparently be most appropriate for use in alignment. It is well-known that errors of fifth order can be influenced by a twelve-pole field. However, a twelve pole field does not result in a satisfactory solution in the event that the twelve pole field is produced at the location of the multipoles, since the elimination of opening errors of fifth order at these locations results in the introduction of additional errors.
It is therefore the underlying purpose of the invention to further develop a corrector of the above-mentioned type in such a fashion that aberrations up to fifth order can preferentially be largely eliminated.